The invention is in the field of transistor amplifier circuits, and relates more particularly to a high-frequency amplifier circuits as well as bias circuits for such amplifier circuits.
Typically, the output stage of such high-frequency amplifiers is a bipolar transistor connected in a common-emitter configuration, biased with either a voltage source or a current source at the base.
In IC technology, a straightforward way of biasing the output stage is to apply a constant-current source at the base. This current source can directly control the quiescent current in the output stage. However, the collector-to-emitter breakdown voltage of the output stage with a current source coupled to the base is lower than the corresponding breakdown voltage with a voltage source coupled to the base. Accordingy, an alternative technique is to coupled a voltage source to the base of the output transistor. The drawback of this technique is that it becomes difficult to control quiescent current by varying the base voltage because of the exponential relationship between base current and voltage.
Another prior-art technique for providing control of the quiescent current in the output stage is shown in U.S. Pat. No. 5,828,269, having a co-inventor in common with the instant application. In that reference, however, the bias stage for the output transistor is an emitter follower in the RF signal path, which is controlled by another constant-current source. Accordingly, the output impedance of the bias stage affects both the emitter follower stage and the output stage, a less than optimum result.
Accordingly, it would be desirable to have a high-frequency amplifier circuit in which a bias circuit provides independent and direct control of the quiescent current in the output stage and the output impedance of a bias stage which is not in the RF signal path (i.e. the bias impedance of the output stage) in order to enhance operating parameters of the high-frequency amplifier circuit.
It is therefore an object of the present invention to provide a high-frequency amplifier circuit in which a bias circuit provides independent and direct control of both the quiescent current in the output stage and the output impedance of a bias stage not in the RF signal path. Such an arrangement permits excellent control of gain and class of operation and also permits the circuit to be adjusted for optimum efficiency and linearity.
In accordance with the invention, this object is achieved by a new high-frequency amplifier circuit which includes an amplifying transistor and a bias circuit coupled to the amplifying transistor, with the bias circuit including a first bias subcircuit for controlling a quiescent current in the amplifying transistor and a second bias subcircuit for independently controlling a bias impedance of the amplifying transistor.
In a preferred embodiment of the invention, the first and second bias subcircuits each include an indepently-adjustable current source and a current mirror.
A further preferred embodiment of the invention is directed to a bias circuit coupled to an amplifying transistor of a high-frequency amplifier circuit of the type described above.
Amplifier and bias circuit configurations in accordance with the present invention offer a significant improvement in that independent and direct control of quiescent current in the output stage and output impedance of the bias stage enable accurate control of amplifier gain and class of operation, while providing good efficiency and linearity.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.